The present invention relates to the field of power systems for marine vessels and is especially useful for marine seismic exploration vessels. More particularly, the invention relates to a secondary power source for storing large quantities of energy and for selectively discharging such energy when a primary power source for the vessel is inoperative.
Seismic exploration methods indicate the physical properties and spatial disposition of geologic formations underlying the earth surface. In land based seismic systems, mechanical vibrators or explosive charges initiate a pressure wave in earth materials. In marine systems, a seismic source array comprising air guns or other pressure source wave generators are towed by a vessel through the water. Source waves from a seismic source travel downwardly through the subsurface formations, and portions of the seismic wave energy are reflected, refracted and transmitted by geologic interfaces between subsurface geologic units. The returning reflected waves are detected by a sensor or sensor arrays located at a known position relative to the energy source. In marine seismic operations, the reflected waves are detected with towed sensors such as hydrophones or bottom cables positioned on the ocean floor.
Seismic sensors in marine systems comprise transducers which convert pressure, velocity or acceleration into electric signals. The output signals are recorded and processed to indicate mechanical, acoustic and structure characteristics of geologic units. For example, the propagation time of a seismic wave reflected from a point is proportional to the depth of the reflection point. Travel times from multiple, spatially diverse source and receiver pairs facilitate construction of maps representing the spatial disposition of subsurface geologic units.
Towed vehicles are typically neutrally buoyant and depend on water flow over control surfaces to maintain position in the water. If the tow vehicle should lose forward motion, the tow vehicle is uncontrolled and may sink. Such event can damage towed arrays and can sink the tow vehicle and the tail buoy marking the array tail-end. Even if the equipment does not sink, the submerged tow vehicle and towed array will comprise a navigation hazard.
Numerous vessel propulsion systems have been developed for marine vessels, including gasoline engines, diesel engines, electric motors, and jet propulsion units as the primary sources of power. Secondary power sources permit vessel operation following failure of the primary power source. Otherwise, power failure can lead to irretrievable loss of the vessel, towed equipment and data acquired by the vessel. Existing secondary power sources for marine vessels typically comprise storage batteries such as silver/zinc batteries or auxiliary fuel-powered propulsion systems. Battery power sources are heavy and require recharge time and maintenance. Auxiliary power systems are subject to the same failure mechanisms experienced by the primary power sources, and may not provide reliable auxiliary power. Conventional batteries and conventional generators provide relatively low storage density for secondary power requirements. Accordingly, a need exists for an improved apparatus for storing and discharging large amounts of energy. The apparatus should be portable and should be capable of sustaining energy discharge over a defined time interval.